Fix projection.clipExtent edge case.

A slight correction to clipLine fixed the problem with points lying
exactly on the clip edge, but in case clipLine(a, b) ever returns false
(no intersections) even though v_=0 and v=1, we treat this as the start
of a line anyway.

d3.js

@@ -2788,7 +2788,7 @@ d3 = function() {
     }
   }
   function d3_geo_clipViewT(num, denominator, t) {
-    if (Math.abs(denominator) < ε) return num < 0;
+    if (Math.abs(denominator) < ε) return num <= 0;
     var u = num / denominator;
     if (denominator > 0) {
       if (u > t[1]) return false;

src/geo/clip-view.js

@@ -134,6 +134,9 @@ function d3_geo_clipView(x0, y0, x1, y1) {
             }
             listener.point(b[0], b[1]);
             if (!v) listener.lineEnd();
+          } else {
+            listener.lineStart();
+            listener.point(x, y);
           }
         }
       }
@@ -197,7 +200,7 @@ function d3_geo_clipView(x0, y0, x1, y1) {
 }
 
 function d3_geo_clipViewT(num, denominator, t) {
-  if (Math.abs(denominator) < ε) return num < 0;
+  if (Math.abs(denominator) < ε) return num <= 0;
 
   var u = num / denominator;